Not sure if this has been noticed before. Doing some trial bookings, LONE3 ex-BUD.

If you choose the AA flight, BUD-JFK as the 1st flight, the fare is quoted in EUR and over 400EUR in fuel fines. It was a simple BUD-JFK-NRT-HEL-MAD-BUD, only BUD-JFK on AA.

Changed the BUD-JFK to BUD-MXP on MA connecting to the AA MXP-JFK. Price is quoted in HUF and no fuel fines listed. Though the largest 'fee' was 29,100 HUF ($133 US) Airline Insurance surcharge, which may be a fuel fine.

My guess is that with the AA/BA/IB JBV, the online booking engine is adding fuel surcharges when using AA as the 1st carrier. Though with BA as the 1st flight, the fuel fines were higher than AA, but not that much more.